Field of the Invention
The present invention relates to photoconductive members sensitive to electromagnetic waves such as light (the light herein is in a broad sense including ultraviolet rays, visible rays, infrared rays, X-rays, .gamma.-rays, etc.).
Description of the Prior Art
Photoconductive materials for constructing photoconductive layers in solid-state image pickup devices and in image forming devices, e.g. electrophotographic image forming members and original-reading devices, are requested to have characteristics such as high sensitivity, high S/N ratio [photo-current (Ip)/dark current (Id)], absorption spectral characteristics fitting the spectrum of the electromagnetic waves to be irradiated, quick photo-responsiveness, desired dark photo-responsiveness, desired dark resistivity, and harmlessness to human body during use, and further for solid-state pickup devices, the ease of after-image treatment. In particular, the harmlessness to human body during use is important for the photoconductive materials to be incorporated into electrophotographic appliances for office purposes.
A photoconductive material recently noticed on the ground of the above points is amorphous silicon (hereinafter designated as a-Si), which is described in for example, German Pat. Offen. Nos. 2,746,967 and 2,855,718 as an electrophotographic image forming member and German Pat. Offen. No. 2,933,411 about application thereof to a photoelectric conversion type of reader.
However, the prior art photoconductive member having a photoconductive layer of a-Si still needs to be improved in overall characteristic including electrical, optical, and photoconductive properties such as dark resistivity, photosensitivity, and photo-responsiveness, environment resistances such as humidity resistance, and stability with time passage.
For instance, when photoconductive members of a-Si type were used as electrophotographic image forming members, attempts to enhance the photosensitivity and simultaneously the dark resistance, in the past, often brought about undesirable effects such that; residual potential was frequently observed during service; fatigue accumulated during repeated operations continued for long hours and this was accompanied by a so-called ghost phenomenon that is the occurrence of an afterimage; and the responsiveness gradually deteriorated during high-speed repeated operations.
In addition, a-Si has a relatively low absorption coefficient for light waves longer than short-side visible light waves. Accordingly, the prior art photoconductive layer made up of a-Si is ineffective in utilizing the energy of the semiconductor lasers practically used today or the longer-wave energy of halogen lamps or fluorescent lamps, when these light sources are used for the irradiation. This is one of the still remaining problems to solve.
Meanwhile, when a large portion of the incident light arrives at the substrate of a photoconductive member without being absorbed by the photoconductive layer, light interference due to multiple reflection will takes place in the photoconductive layer if the substrate has a high reflectance for the light incident thereupon through the photoconductive layer. This interference is a factor in causing the "unfocused image". This effect becomes more remarkable as the irradiation spot is made smaller for the purpose of improving the resolution. This is a significant problem in particular when a semiconductor laser is employed as a light source.
Moreover, when the photoconductive layer of a-Si is formed to contain hydrogen atoms or halogen atoms such as fluorine or chlorine for improving electrical and photoconductive properties thereof or boron, phosphorus, or some other atoms for controlling the electroconductive type, a problem arises occasionally in the electrical or photoconductive properties of the resulting layer depending upon the manner of incorporating these atoms. For instance, the problem is that photocarriers produced by light irradiation have insufficient lifetimes in this layer or that the injection of electric charge from the substrate side of the dark area is not sufficiently impeded.
Furthermore, in the formation of the photoconductive layer thicker than ten and several .mu., a phenomenon such as the lifting or peeling of the layer from the substrate or the cracking in the layer is liable to develop with the time of leaving the product in the air after withdrawal thereof from the vapor-deposition chamber used for the layer formation. This phenomenon is remarkable in particular when the substrate is of drum type commonly used in the field of electrophotography. Thus the photoconductive member involves problems to solve in respect to the stability with passage of time.
Accordingly it is necessary to improve characteristics of a-Si material itself and on the other hand, to design photoconductive members employing it, so as to solve all the problems noted above.